1. Field of Invention
The present invention relates to cellular communication; and more particularly to a method and apparatus for mobile assisted handoff.
2. Description of Related Art
A cellular communication system includes several base transceiver stations, each of which transmits and receives channels of RF information throughout a predetermined coverage area referred to as a cell. The outer boundary of a cell is determined not only by the effective radiated power of the base transceiver, which determines the maximum range at which a mobile unit is able to receive; but also, by the effective radiated power of the mobile telephones, which determines the maximum range at which the base station is able to receive. The base transceiver stations (BTS) determine their corresponding cell's handoff boundaries by setting an RF signal threshold value which must be met or exceeded by a mobile unit entering the cell in order to transfer an on-going call to that cell.
In typical cellular installations, each BTS has several scanning receivers that periodically, or on command, measure the RF signal strength of selected traffic channels in order to determine the range of active mobile telephones relative to the transceiver of the cell in which the mobile telephone is located, referred to herein as a serving cell, and the proximity of the mobile unit to neighboring cells, which may overlap the serving cell.
In cellular communication systems with digital capability, the mobile unit also has a scanning receiver for measuring the signal strength of neighboring cells, and reporting this signal strength to the base station.
In such digital systems, at a certain drop in the signal level, the base station initiates mobile assisted handoff by sending a measurement order to the mobile unit in accordance with standard protocol. This measurement order may accommodate up to twelve neighboring cell RF channels, for example, to be measured by the mobile unit. Once the mobile unit reports the signal strengths of the twelve requested RF channels, the base station at the serving cell determines the best neighboring cell from the measurements reported by the mobile unit and initiates the handoff process.
In the event that none of the cells reported by the mobile unit meet the handoff criteria, the measurement order is repeated until the mobile unit has reached a location where at least one of the scanned cells meets the criteria for handoff.
Since the goal of the base station is to handoff to the best neighboring cells, where there are more neighboring cells in the measurement order list than the maximum number of cells able to be scanned at one time by the mobile unit, it is only by chance that the best neighboring cell is included in any of the measurement order to the mobile unit.
This, of course, results in substandard communications, particularly where the mobile unit is closer to a cell other than the one having the traffic channel to which the mobile unit was handed off. It also results in excessive handoffs because the signal strength of the cell to which the mobile unit is handed off may have a signal strength close to the threshold level of another neighboring cell; and also may result in an unbalanced distribution of calls on neighboring cells.
In light of the foregoing, there is a need for an apparatus and method of handing off an on-going communication in a mobile assisted handoff (MAHO) that overcomes one or more of the disadvantages of prior arrangements.